This invention relates to a special sound effects circuit for use with an electrical musical instrument and, more particularly, to such a circuit in which the sound effects are achieved by controlling both the rate of phase shift modulation and the average amount of phase shift imparted to an input signal.
Numerous circuits are known which operate on an input audio signal from a musical instrument to produce different sound effects pleasing to the ear. In particular, a number of circuits are designed to simulate at least some of the low rate frequency modulation and amplitude modulation effects developed by a mechanical system such as that disclosed in U.S. Pat. No. Re. 23,323 issued to D. J. Leslie in which a speaker is rotated about a vertical axis at a rate on the order of 1 to 15 revolutions per second.
Some such circuits are shown in U.S. Pat. No. 3,644,657 issued Feb. 22, 1972, to Francis A. Miller. In FIG. 10 of this patent, an input signal is phase shifted by a network producing at least 720.degree. of phase shift and the input signal and phase shifted signal are then summed together to produce an output signal which when converted to sound has some of the sound characteristics of a rotary speaker system.
A circuit is shown in FIG. 7 of this patent which is apparently designed to enable an operator to simulate the starting and stopping characteristics of a rotating speaker. The magnitude of a control signal which establishes the modulation rate may be selectively varied by manually operable controls at or near the musical instrument to increase and decrease the modulation rate to respectively simulate the sound effects produced as the rotary speaker velocity is increased and decreased. However, the rate of change of the modulation rate is apparently not preselected and thus requires more than a simple on-off operation by the user. Further, the frequency response of the phase shifting network remains unaffected by the control signal such that, unlike a rotary speaker system, a flat response curve to the midrange of the audio frequency spectrum at the lower modulation rates is not achieved.